1. Technical Field
This invention relates to electronic circuit designs for column drivers for an active matrix (thin-film transistor) liquid crystal display.
2. Description of Related Art
With recent progress in various aspects of active matrix (thin-film transistor) liquid crystal display technology, the proliferation of active matrix displays has been spectacular in the past several years. In an active matrix display, there is a gate comprised of one transistor or switch corresponding to each display cell. An active matrix display is operated by first applying select voltages to a row electrode to activate the gates of that row of cells, and second applying appropriate analog data voltages to the column electrodes to charge each cell in the selected row to a desired voltage level.
Column drivers are very important circuits in the design of an active matrix display panel. The column drivers receive digital display data and control and timing signals from a display controller chip, convert the digital display data to analog voltages, and drive the analog voltages onto column electrodes of the display.
To convert the digital display data to the analog voltages, existing column drivers use either a resistor-string based digital-to-analog converter (DAC) or a capacitor-based DAC. This invention concerns improving the design of a column driver that uses a resistor-string based DAC. The resistor-string DAC in a conventional column driver includes a single resistor string in combination with multiple decoders (one for each column electrode to be driven). The resistor string DAC interpolates voltages between analog reference levels that are provided to the column driver.
In a column driver using a resistor-string DAC, there are several known techniques for driving the analog voltages onto the column electrodes. The first known technique is a "direct drive" technique in which analog voltages from the output of the resistor string DAC is directly driven to the columns. The second known technique is to use an "ordinary buffer" to buffer the output from a resistor-string DAC to drive the column electrodes. The third known technique is to use a "timed buffer" that is activated in a timely fashion.
In the direct drive technique, the column electrodes are driven directly from the resistor string. This, in theory, may result in accurate voltages because of the inherent linearity of the resistors in the resistor string. However, because the resistance of the resistors must be small in order to drive the column capacitance with sufficient rapidity (since the time constant of the circuit is proportional to the resistance), this direct drive system results in high consumption of power at the resistor string (since power is inversely proportional to the resistance) and hence less current available to drive the columns. In addition, the direct drive technique requires the use of a powerful external reference amplifier circuit in order to increase the drive power on the analog reference levels provided to the resistor string so that the capacitance of the display panel may be driven within a required time. With an external reference amplifier circuit, however, a voltage drop may occur in the printed circuit board (PCB) between the amplifier circuit and the column driver chips because a large DC current must be provided to the resistor-string DAC while AC current must be provided to charge the substantial capacitance of the display panel. Finally, if too many of the decoders select the same tap (between two resistors in the resistor string) such a condition may be aggravated.
In the ordinary buffer technique, the disadvantages of the direct drive system are overcome by interposing analog output buffers in between the column decoders and the column electrodes. In the ordinary buffer technique, because the resistor string does not directly drive the column capacitance, large resistor values may be used to reduce power consumption at the resistor string. In addition, the presence of the analog output buffers eliminates the need for the high current output reference amplifier circuitry, and without the high current output reference amplifier circuitry the problems due to voltage drops in the PCB and in the decoder circuitry can be minimized.
Nevertheless, the ordinary buffer technique has several disadvantages. Since each column driver typically supports three hundred columns and each column requires a buffer, a large number of buffers are needed. In addition, these buffers consume a large amount of power and are not power efficient. Finally, the voltage offset of these buffers causes voltage inaccuracy.
In the timed buffer technique, each analog output buffer in the ordinary buffer system is replaced with a timed buffer circuit that includes a buffer, a switch, and timing and control circuitry. In a first "predrive" stage, the switch is turned off, and the buffer drives the column capacitance without drawing substantial current from the resistor string. In a second "precision drive" stage, the buffer is turned off, and the switch is turned on so that the column capacitance is driven to its final value directly from the resistor string.
The timed buffer technique reduces the large power consumption at the buffers because after the first stage the buffers are turned off. In addition, since the final value is driven by the resistor string, there is no voltage offset.
However, the timed buffer technique has disadvantages. It requires additional control and timing circuitry to control and time its two-stage operation. Furthermore, it still requires a large number of buffers since one is needed for each column.